1. Field of the Invention
The present invention relates to a differential current-switch circuit, a D/A conversion circuit employing this differential current-switch circuit and a method of transient response reduction.
2. Description of the Related Art
In a system for electron beam exposure, since drawing is performed by scanning an electron beam onto a wafer, the exposure time is long. Thus, it is necessary to improve the speed of the electron beam scanning performed by a deflector. To satisfy this requirement, a D/A converter 10, shown in FIG. 7, which is cap able of high speed operation, is employed. This D/A converter 10, which is a current-drive ladder type converter, is provided with a resistance ladder circuit 11 having resistors whose resistance values are R and 2R and differential current-switch circuits 20, structured identically to one another, the number of which is equal to the number of input bits of the D/A converter 10. The current switch circuits 20 are connected between a ground wiring GND at 0 V and a power-supply wiring VEE with a negative potential of, for instance, -8 V. The current switch circuit 20 comprises a constant current source 21, transistor switches 22A and 22B such that when one is on, the other is off, and resistors 23A and 23B respectively connected in series with the transistor switches 22A and 22B. The resistor 23B is one viewing the resistance ladder circuit 11 from line A--A and its resistance value is 2R/3, and this is the same for each of the current switch circuits corresponding to the individual bits. Therefor, the resistance value of the resistor 23A is also set to 2R/3 and this is the same for each of the current switch circuits. This D/A converter 10 outputs a current I, which is expressed as EQU I.sub.out =I.sub.c (S0.multidot.2.sup.0 +S1.multidot.2.sup.1 +S2.multidot.2.sup.2 + . . . +S(n-1).multidot.2.sup.(n-1))
in response to bits (S0, S1, S2, . . . , Sn-1) of an input value.
The current I is converted to a voltage by a current/voltage conversion circuit 30 comprising a operational amplifier 31 and a resistor 32.
A first structural example of the current switch circuit 20 is shown as 20X in FIG. 8. The transistors shown in FIG. 8 are all NPN type transistors.
Since the current flowing through the resistor 23B is used for output of the D/A converter 10, the transistor switch 22B is constituted with a transistor 22B1 and a transistor 22B2 which have the Darlington connection in order to turn it off/on at high speed and with a high degree of accuracy.
The constant current source 21 is provided with transistors 211 and 212 which have the Darlington connection, a resistor 213, which is employed in order to detect the current and convert it to a potential VX, and a comparator 214 for performing feedback control so that the detected potential VX is made equal to a reference potential V1 in order to stabilize the current fluctuation that occurs when the transistor switches 22A and 22B are turned on/off. The Darlington connected transistors 211 and 212 are employed in order to stabilize the current fluctuation at higher speed by a large gain.
A pair of ECL complementary signals are respectively provided to the bases of the transistor switches 22A and 22B2, and the difference between its `high` state VH and its `low` state VL may be, for instance, 1.6 V.
When the base of the transistor switch 22A is at `high` state VH and the base of the transistor switch 22B is at the `low` state VL, the transistor switch 22A is on and the transistor switch 22B is off, resulting in that a current flowing through the resistor 213 is almost equal to the current (VX-VEE)/R flowing through the resistor 23A. At this time, the emitter potential of the transistor switch 22A is approximately at (VH-0.7)V.
When the base of the transistor switch 22A is at `low` state VL and the base of the transistor switch 22B is at `high` state VH, the transistor switch 22A is off and the transistor switch 22B is on, resulting in that the current through the resistor 23B is equal to that described above. At this time, The emitter potential of the transistor 22B1 is at (VH-1.4)V and the potential at the node of connecting the emitters of the transistor switches 22A and 22B1 becomes lower than that in the case described above by 0.7 V.
In the differential current-switch circuit 20X, switching over between the switches may be performed by changing the base potential of only one of the transistor switches 22A and 22B1, leaving the base potential of the other fixed. However, in that case, the fluctuation amplitude of the potential at the node of connecting the emitters is larger than the case described above and, therefore, it is not desirable.
As for the DC characteristics, the detected potential VX is made equal to the reference potential V1. However, when the emitter coupled potential VE falls or rises, transient response occurs in the feedback control performed by the comparator 214, causing the detected potential VX to oscillate, which results in oscillation of the current flowing through the resistor 23B. Because of this, as shown in FIG. 10, the output current I from the D/A converter 10 undergoes a transient oscillation at a step change. Since its oscillatory waveform varies depending upon differences in the characteristics of the transistor switches 22A and 22B, it varies, generally speaking, depending upon the input values of the D/A converter 10 before and after the change thereof.
Thus, for the drive circuit for the deflector described above, a method that all the oscillatory waveforms of the output current I corresponding to the input values of the D/A converter 10 are stored in a memory and waveform data are read out from addresses corresponding to an input value of the D/A converter 10 to correct the current I by inverting the sign of the waveform data has been proposed.
In order to reduce the fluctuation of the potential of the connected emitter node that occurs when switching over current paths, a differential current-switch circuit 20Y of FIG. 9 has been employed. In this circuit, a buffer circuit 24 is connected between the connected emitter node and the constant current source 21. In the buffer circuit 24, a transistor 241 and a transistor 242 have the Darlington connection and a potential V2 from a constant voltage source 25 is applied to the base of the transistor 242 so that the buffer circuit 24 can also operate as a constant current source. A measurement result indicates that the fluctuation of the emitter potential of the transistor 241 can be reduced to 7 mV, which is approximately 1/100 of the emitter coupled potential VE.
Since the Darlington connection is employed to achieve fast response at the constant current source 21 and its current amplification factor hoe is comparatively large, transient response is likely even when the fluctuation is slight. Since minute patterns are exposed in an electron beam exposure system, a high accuracy D/A converter 10 with approximately 18 bits is employed and, as a result, even if the fluctuation is at approximately 7 mV as described above, the oscillation cannot be disregarded. In order to further reduce this oscillation, buffer circuits 24 may be connected over two stages. However, this will increase the number of transistor stages between power-supply wirings, which will result in an increase in the power-supply voltage required and also result in an increase in the characteristic change due to heat arising from the increase in power consumption, further presenting a cause of reducing the operating speed.